1. Field of the Invention
This invention relates generally to wireless communications and, more particularly, to the operation of a Radio Frequency (RF) transceiver within a component of a wireless communication system.
2. Description of the Related Art
The structure and operation of wireless communication systems are generally known. Examples of such wireless communication systems include cellular systems and wireless local area networks, among others. Equipment that is deployed in these communication systems is typically built to support standardized operations, i.e., operating standards. These operating standards prescribe particular carrier frequencies, modulation types, baud rates, physical layer frame structures, MAC layer operations, link layer operations, etc. By complying with these operating standards, equipment interoperability is achieved.
In a cellular system, a regulatory body typically licenses a frequency spectrum for a corresponding geographic area (service area) that is used by a licensed system operator to provide wireless service within the service area. Based upon the licensed spectrum and the operating standards employed for the service area, the system operator deploys a plurality of carrier frequencies (channels) within the frequency spectrum that support the subscribers' subscriber units within the service area. Typically, these channels are equally spaced across the licensed spectrum. The separation between adjacent carriers is defined by the operating standards and is selected to maximize the capacity supported within the licensed spectrum without excessive interference. In most cases, severe limitations are placed upon the amount of adjacent channel interference that maybe caused by transmissions on a particular channel.
In cellular systems, a plurality of base stations is distributed across the service area. Each base station services wireless communications within a respective cell. Each cell may be further subdivided into a plurality of sectors. In many cellular systems, e.g., Global System for Mobile Communications (GSM) cellular systems, each base station supports forward link communications (from the base station to subscriber units) on a first set of carrier frequencies, and reverse link communications (from subscriber units to the base station) on a second set of carrier frequencies. The first set and second set of carrier frequencies supported by the base station are a subset of all of the carriers within the licensed frequency spectrum. In most, if not all, cellular systems, carrier frequencies are reused so that interference between base stations using the same carrier frequencies is minimized and system capacity is increased. Typically, base stations using the same carrier frequencies are geographically separated so that minimal interference results.
Both base stations and subscriber units include RF transceivers. Radio frequency transceivers service the wireless links between the base stations and subscriber units. The RF transmitter receives a baseband signal from a baseband processor, converts the baseband signal to an RF signal, and couples the RF signal to an antenna for transmission. In most RF transmitters, because of well-known limitations, the baseband signal is first converted to an Intermediate Frequency (IF) signal and then the IF signal is converted to the RF signal. Similarly, the RF receiver receives an RF signal, down converts it to IF and then to baseband. In other systems, the received RF is converted directly to baseband.
Radio receivers typically include several circuits that each provides an amount of gain to the received signals. For example, mixers and low pass filters each often provide gain. Because, however, the signal strength of a received signal can vary significantly, there is a need for amplifiers within the radio receiver whose gain level is adjustable. Programmable amplifiers often vary a feedback resistance value to adjust gain. Typically, a low noise amplifier is used to amplify the received signal prior to mixing it with a local oscillator. The level of amplification provided, however, must often be adjusted to compensate for fluctuations in received signal strengths.
One problem with using a mixer and an active low pass filter, however, is that an amplifier used within a low pass filter typically comprises an Op Amp that provides a fast response but without the ability to drive current through a load. Accordingly, it is possible to provide an amplifier in a source follower configuration on the output stage of the Op Amp to drive current through the load. One problem with using a source follower configuration, however, is that the output voltage is reduced due to voltage drops from the gate to source of the source follower. This becomes a significant problem in CMOS devices because the supply voltage is typically only 3 volts DC. Thus, what is needed is a circuit that provides a fast response, can drive current through a load and does not reduce any of the output voltage potential.